1. Field of the Invention
The present invention relates to semiconducting ceramic, particularly to a semiconducting ceramic having a negative resistance-temperature coefficient, to a semiconducting ceramic element using the semiconducting ceramic, and to a method for producing the semiconducting ceramic.
2. Description of the Related Art
Semiconducting ceramic elements (hereinafter referred to as NTC elements) having high resistance at ordinarily temperature and having negative temperature coefficients of resistance (hereinafter referred to as NTC characteristics) so that the resistance decreases as the temperature increases are known. The NTC elements are used for suppressing the rush current, for delaying motor start and for protecting halogen lamps, in consideration of the above characteristics.
An NTC element for suppressing the rush current absorbs an initial rush current and suppresses an overcurrent in a circuit when a switching electrical power source is turned on. The NTC element is subjected to self-heating so as to decrease the resistance thereof. Thus, electrical power consumption is reduced at the stationary state.
An NTC element for delaying motor start applies a decreased voltage to motor terminals in an initial state to suppress the rotation of a motor mounted in a gear unit in which lubricant oil is supplied after the motor starts. The NTC element is self-heated so as to decrease the resistance thereof. Thus, the rotation of the motor gradually increases and reaches a stationary state.
Semiconducting ceramic having NTC characteristics for NTC elements contain spinel-type compound oxides comprising transition metal elements, such as Mn, Co, Ni, and Cu. For example, V. G. Bhide and D. S. Rajoria (Phys. Rev. B6, [3], p. 1072 (1972)) disclose a lanthanum cobalt oxide having NTC characteristics in which the B constant shows temperature dependence and increases at higher temperature. Japanese Unexamined Patent Application Publication No. 7-176406 discloses a compound oxide comprising La.sub.x CoO.sub.3 as the major component, wherein 0.600.ltoreq.x.ltoreq.0.999 and an oxide of Si, Zr, Hf, Sn, W, Mo or Cr. The compound oxide has a resistivity of 20 .OMEGA. or less at ordinary temperature and a B constant of 3,200 K or more.
It is known that semiconducting ceramics comprising lanthanum cobalt oxides represented by La.sub.x CoO.sub.3 are spontaneously degraded when lanthanum is present in excess of cobalt (x&gt;1). Excess lanthanum is precipitated in the grain boundaries of the ceramic and is modified from oxide to hydroxide by moisture absorption. As a result, the excess lanthanum swells and causes degradation of the semiconducting ceramic. Thus, the semiconducting ceramic comprising La.sub.x CoO.sub.3 as the major component are prepared so that Co is present in excess of La (x&lt;1).
In the semiconducting ceramic containing excess cobalt with respect to lanthanum, cobalt oxides, such as CoO and Co.sub.3 O.sub.4, are also present in addition to lanthanum cobalt oxide LaCoO.sub.3. LaCoO.sub.3 has a low resistivity of 20 .OMEGA. at ordinary temperature, whereas Co and Co.sub.3 O.sub.4 have a high resistivity of 1 M .OMEGA. or more. When such a low-resistivity material and a high-resistivity material are simultaneously present in the semiconducting ceramic, it is difficult to control the production steps for achieving the same resistivity for individual products, compared to spinel-type compound oxides comprising transition metal elements.